Provide clear error message, when package cannot be found

[isabelizimm]

Hey there--

Not sure what the change is, but with quartodoc==0.3.4 and griffe==0.29.1 (both of the latest versions), my doc builds are failing with an error of:

    raise WorkaroundKeyError(e.args[0])
quartodoc.builder.utils.WorkaroundKeyError: VetiverModel

See full logs here

My docs pass when I have quartodoc==0.3.4 and griffe==0.25.1, so this is not blocking me, I'll just use that configuration of dependencies in the meantime 😄

[machow]

I think this is related to mkdocstrings/griffe#172

The underlying pth files (which are used for editable installs) are using a funky format, that is causing griffe not to be able to find vetiver. I'll submit a PR there in a sec!

[machow]

Since this error was about the underlying package not being found, it was confusing that the message was about a specific entry. Let's make sure the message that pops up here says something like, "package X can't be found" (e.g. by an early check, or drilling down when the error occurs).